Box Sign has an integration for Salesforce.com but it really does not save much real effort because the integration is not complete. Box Sign really needs to integrate with Salesforce's Files object as Salesforce.com is the system of record and therefore needs the file Signed version of documents.

In an ideal world, Box would be able to both pull documents from the Files object and save the Signed Document back to the Files object in Salesforce.com (but while you are at it, you might as well make it configurable to Documents, Files, or other file repositories in Salesforce). Otherwise the process is download the file from Salesforce, upload the file to the Box widget in Salesforce, set up the document for signature, send to customer to sign, once signed, download from Box, upload back to Salesforce.com Files object. As you can see this is a ton of steps and makes it far less useful as a Integration, you might as well just use Box.com in this case (so what is the point of the integration at that point).